Reply from address in Mountain Lion Mail
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I have a folder in which I keep all my WAMUG mail. The mail in there is addressed to my wamug address and up until Mountain Lion, when I clicked reply, the reply was sent from my wamug address. Now however, it defaults to my default address for new mail. If you're not careful, someone sending you an email to your work address could get their reply from your personal address!
This has been widely reported and it's possible to find a script online to change this behaviour but Apple have yet to address it ;-)